Zebra Mussels were found in Marimo Moss Balls sold in Montana. Any moss balls recently purchased should be taken out of the water, put in a plastic bag in the freezer overnight. Aquarium water should be treated as contaminated. Aquarium water should not be dumped down household drains or into any water body without sterilization. USFWS suggests removing fish and other organisms from aquarium water, and treating water with bleach for 10 min. It is also important to sterilize aquarium decorations.
For more information on what to do: Destroy! Don't Dump--USFWS